1. Field of the Invention
The present invention relates to a surface inspection apparatus which employs a microscope and a television camera.
2. Prior Art
In a microscopic surface inspection apparatus, it is necessary to measure the distances between an objective lens and an object to be inspected (hereinafter referred to as "object") each time when the small visual field of a microscope are sequentially shifted in order to inspect the surface of the object in a reasonable area. That is the viewing field of a microscope which is small in accordance with the largeness of magnification of the microscope. In this case, a focus unmatching problem arises due to the fact that the focal depth of the microscope becomes smaller with the increase of magnification, as a result, a vivid focus must be hardly established to the object surface, because of uneven distances between the objective lens and the object surface by the reason that the thickness of the object is not uniform or the entire bottom surface of the object does not contact with the slide glass of the microscope.
Conventionally, automatic focusing device provided with an air micrometer system has been used to solve the above problem. In this type of a device, air is jetted through a space between an objective lens and the surface of the object so as to detect the pressure of the jetted air. Thereafter, the objective lens or the object is moved vertically so that the detected air pressure becomes the sam value with the air pressure predetermined in accordance with a predetermined distance between the object and the objective lens. This method, however, is effective only for maintaining the distance between the object surface and the objective lens to be constant. According to this method, a precise focal length must be adjusted by visually monitoring. Further, the distance between the object and the objective lens cannot be correctly measured when the object is covered with a transparent film, due to the existence of the thickness of the film.
Japanese Laid-Open Patent Publication No. 28340 of 1977 discloses a method for automatically adjusting a focal point in which a focal point is adjusted by obtaining a point where the rate of change in the magnitude of a video signal is maximum at the edge of an object of the outline of a pattern thereof. This method has also a disadvantage that a focus adjustment cannot be made in the case of a sheet-like object such as a sheet of blank paper or a piece of cloth which has no visible outline, or so wide object that its visible outline is out of the visual field of a microscope.